Secondary sealing element

ABSTRACT

A secondary sealing element comprises a base body ( 12 ) made of a synthetic material and comprising a base portion ( 13 ) and a seal portion ( 14 ) which each comprise coaxially aligned, axially adjacent through bores ( 17, 31 ) for the passage of a component, and an annular disc element ( 25 ) accommodated in the base portion and comprising a through bore ( 30 ) coaxially aligned with the through bores in the base and seal portions and being made of a material which differs from that of the base body. In the unloaded state, the through bore ( 30 ) of the annular disc element ( 25 ) has a radial dimension d which is greater than that D 2  of the through bore ( 31 ) in the seal portion ( 14 ) and smaller than that D 1  in the base portion ( 13 ) of the base body ( 12 ). The annular disc element consists of a carbon material. An important field of application for the secondary sealing element is that of mechanical face seals for sealing relatively moveable components.

The invention relates to a secondary sealing element in accordance withthe preamble of claim 1.

The invention relates in particular to a secondary sealing element foruse with a mechanical face seal device which is designed to be used forthe purposes of sealing gaseous media at high to very high pressureswhere there is a danger of the material of the secondary sealing elementflowing or being extruded into the narrow gaps requiring sealing formedbetween adjacent components whose movement relative to one another mustbe ensured. A secondary sealing element has already been proposed(DE-U-29518119) wherein, for the purposes of preventing such extrusion,a rigid supporting element is embedded in a base body consisting of aflexible synthetic material in such a way that it practically covers anentrance to the gap requiring sealing whereby the material of thesecondary sealing element is prevented from entering into the gap. Therigid supporting element does not perform a sealing function. Rather,this function is retained exclusively by the base body of the knownsecondary sealing element. Although indeed the penetration of thematerial of the secondary sealing element into the gap requiring sealingcan be effectively prevented by these known measures, nevertheless highpressures, especially in the presence of high temperatures, cannot bereliably sealed using the known secondary sealing element i.e. there isa danger that under such operational conditions the freedom of movementof the component concerned, which may be a biased seal ring, cannot beensured. On the other hand, free movement of the seal ring in an axialdirection is an essential prerequisite for its sealing effect and foradequate operational reliability especially for the sealing of gases.Moreover, lack of mobility can have substantial harmful effects.

The object of the invention is to provide a secondary sealing element ofthe type mentioned hereinabove which is suitable for use at normal tovery high pressures and in the presence of gaseous media requiringsealing by ensuring adequate mobility of components which are moveablerelative to one another and which are to be mutually sealed by thesecondary sealing element.

This object is achieved by means of a secondary sealing elementconsisting of a base body of a synthetic material and comprising a baseportion and a seal portion. The base and seal portions each includes athrough bore for the passage of a component, said through bores areaxially adjacent and coaxially aligned with each other. An annular discelement is accommodated in said base portion and includes a through borecoaxially aligned with the through bores in the base and seal portions.The annular disc element is made of a material which differs from thatof the base body. According to the invention, the through bore of theannular disc element has a radial dimension which, in the unloadedstate, is greater than that of the through bore in the seal portion andsmaller than that in said base portion of the base body, and in that thematerial of the annular disc element comprises a carbon material. At lowpressures of the medium, there is practically no load on the annulardisc element and the seal portion of the base body consisting of theflexible synthetic material performs the sealing function practicallyalone. The sealing forces arising thereby can easily be optimised inregarding to adequate movement of the component concerned (seal ring).By contrast, at higher pressures of the medium, the sealing function isshifted more and more onto the ring seal element which is now pressedinto sealing engagement with the surface requiring sealing. A gapbetween the components requiring sealing is thereby closed at the sametime. Because of the good tribological properties of the carbonmaterial, the movement of the component concerned (seal ring) ismaintained to an adequate extent despite the firm engaging relationshipbetween the ring seal element and the surface. Under high pressureconditions, the load on the seal portion of the base body may berelieved entirely. In accordance with a further development of theinvention, the secondary sealing element can be provided in a recess inan end face of the base portion and project axially beyond the face. Thesecondary sealing element thereby simultaneously produces a radial andan axial sealing effect.

The invention will be explained in more detail hereinafter with the aidof an embodiment and the drawing. Wherein:

FIG. 1 is a longitudinal sectional view of a mechanical face seal deviceshown in a state of mounting on a shaft of a apparatus and having asecondary sealing element in accordance with an embodiment of theinvention, and

FIG. 2 in an enlarged sectional detailed view the secondary sealingelement of FIG. 1,

Although the invention is described hereinafter in connection with apreferred use thereof as a secondary sealing element for a mechanicalface seal device, it is not limited to this field of application.Rathermore, it can also be used to advantage where there is a dangerthat seals made of elastomeric material usually used for sealingrelatively moveable components could be destroyed at higher pressuresand/or temperatures.

As is shown in FIG. 1, the mechanical face seal device comprises a pairof seal rings 3, 4 of which one, namely, the seal ring 3, is heldstationary with respect to a housing 1, whilst the other seal ring 4 isconnected to a shaft 2 for common rotation therewith. In the housing 1,there is a biasing means 6 acting on a thrust ring 5 that is moveableaxially in the housing 1 and upon which the seal ring 3 is axiallysupported so that the seal ring 3 is pressed against the rotary sealring 4 by the bias force exerted by the spring means 6. The seal ring 4is sealed in suitable manner, e.g. by means of an O-ring 9 seated in agroove of a driver housing 7. A tube-like driver ring 8 is provided onthe shaft 2 between the seal ring 4 and a mounting attachment (not shownin detail in the drawing) in order to transfer the rotation of themounting attachment to the seal ring 4. The previously describedconstruction of a mechanical face seal device is known to the skilledperson so that there is no need to go into further detail.

For the purposes of sealing the thrust ring 5 and thus the stationaryseal ring 3 with respect to the housing 1, there is provided a secondarysealing element which bears the general reference number 10 in FIG. 1and is arranged in an L-shaped shouldered seating bore 11 in the thrustring 5.

FIG. 2 shows the secondary sealing element 10 on an enlarged scale. Asshown therein, the secondary sealing element 10 comprises an annularbase body 12 having an L-shaped cross section and consisting of asuitable flexible synthetic material such as polytetrafluoroethylene(PTFE). The base body 12 is composed of a base portion 13 and a sealportion 14 which are axially adjacent and represent integral componentsof the base body 12. The base portion 13 has an essentially rectangularsolid cross section which is axially bounded by an outer end face 15facing the seal ring 3 and an inner end face 16 facing the thrust ring 5and contains a through bore 17 having a radial dimension D₁ throughwhich a sleeve 18 extends that is mounted on the housing 1 in sealingmanner. The thrust ring 5 and the seal ring 3 are seated on the sleeve18 in longitudinally displaceable manner.

The seal portion 14 projects axially from the inner end face 16 of thebase portion 13 close to the through bore 17. The seal portion 14comprises a pair of parallel, radially spaced web elements 19, 20 whichextend axially and delimit an annular space 21 therebetween. Sealingsurfaces 22, 23 are provided on ball type calotte-shaped protrusions onthe outer surfaces of the web elements 19, 20.

The radially inner sealing surface 23 of the seal portion 14 defines athrough bore 31 having a radial dimension D₂ which is smaller than thatof the through bore 17 in the unloaded state of the secondary sealingelement 5.

As can also be taken from FIGS. 1 and 2, a spring element 32 in the formof a U-shaped expansion spring can be arranged in the annular space 21for spreading the web elements 19, 20 radially apart from each other sothat the sealing surfaces 22, 23 are biased so as to rest firmly againstthe neighbouring surfaces requiring sealing in a predeterminable manner.

An annular disc element 25 of rectangular cross section is accommodatedin a recess 24 in the outer end face 15 of the base portion 13 in such away that its outer axial end face 26 projects axially beyond the endface 15 of the base portion 13 by a suitable small amount. The annulardisc element 25 could also be mounted externally on the base portion 13.The inner axial end face 27 of the annular disc element 25 rests insealing manner against a neighbouring surface of the recess 24. At leasta partial surface portion as indicated by 28 of the end face 26 is insealing engagement with a facing end face 29 of the seal ring 3.

One feature of the ring seal element 25 is that it is formed of a carbonmaterial. This material is characterised on the one hand by goodelasticity and inherent stability and hence sealing properties, and, onthe other hand, the thermal expansion behaviour thereof correspondsessentially to that of relevant carbide hard materials such as tungstencarbide WC. A suitable carbon material can correspond to that used forforming seal rings of mechanical face seals and is described withfurther cross references in Burgmann Lexikon, ABC der Gleitringdichtung,self published 1988, page 125. Particularly preferred is a so-calledsynthetic coal, see Burgmann, page 133, loc cit., which may beimpregnated with synthetic resin or another suitable material such asantimony.

Another feature of the ring seal element 25 is that it contains athrough bore 30 through which the sleeve 18 can be inserted withsuitable play in the unloaded state of the ring seal element 25. Thethrough bore 30 is coaxial with the through bore 17 of the base portion13 and has a radial dimension d which is smaller than the radialdimension D₁ of the base portion 13 in the unloaded state of thesecondary sealing element 10. A further feature of the ring seal element25 is that the radial dimension d is larger than that D₂ of the throughbore 31 of the seal portion 14 in the unloaded state. As can also betaken from FIGS. 1 and 2, the pressure of a medium requiring sealing,wherein this is preferably, but not exclusively, a gas, is effectiveboth radially and axially on the ring seal element 25 in that the mediumcan penetrate into the gap (shown enlarged to an exaggerated extent inthe drawing) between the outer periphery of the ring seal element 25 andthe inner periphery of the recess 11 and also between the seal ring 3and the end face 26. As a consequence thereof, the ring seal element 25experiences a radial upsetting which becomes greater as the pressure ofthe medium increases. Thus, as the pressure of the medium increases, thering seal element 25 is pressed into ever firmer sealing engagement withthe neighbouring surface of the sleeve 18 in order to seal the radialgap s between the seal ring 3 and the sleeve 18. The seal portion 14 isthereby practically freed of pressure. By contrast, for smallerpressures of the medium, the sealing is effected primarily by the sealportion 14 due to the engagement between the radially inner sealingsurface 23 thereof and the surface of the sleeve 18 since here, thecontact pressure on the ring seal element 25 is not sufficient to obtainan adequate sealing effect due to the above-mentioned radial dimensionsof the through bores 30, 31. The radial bias force with which thesealing surface 23 is pressed against the surface of the sleeve 18 canbe set in such a way that the axial movement of the thrust ring is notsubstantially impaired, i.e. defined radial sealing forces are presentwhich do not cause an unwanted “hang-up effect” at low pressures of themedium. This freedom of movement is also ensured at high pressures ofthe medium because of the good tribological properties of the carbonmaterial from which the ring seal element 25 is formed. An effect canlikewise be had on the freedom of movement of the ring seal element 25by suitably dimensioning the axial dimensions thereof. At the same time,the ring seal element 25 prevents the danger of a flow or extrusion ofthe material of the base body 5 into the gap s under high pressureconditions.

The sleeve 18 should preferably consist of a wear-resistant hardmaterial such as tungsten carbide WC which has a similar coefficient ofthermal expansion to that of the carbon material of the ring sealelement 25 in order to prevent thermally induced internal stresses inthe ring seal element 25.

The invention was described hereinabove on the basis of an embodimentwherein the ring seal element can simultaneously produce an axial and aradial sealing effect. However, the invention is not restricted to suchan arrangement, but rather, it also encompasses an arrangement whereinthe ring seal element can only fulfil a radial sealing function in thatit is arranged entirely within the contours of the base body. Theconstruction of the seal portion is not limited to the provision ofradially expandable web elements. The seal portion could also be in theform of an e.g. labyrinth seal or a simple lip seal. Finally, thesecondary sealing element according to the invention can be provided foruse in connection with both the stationary and the rotary seal ring.Although the use of a carbon material is preferred, other suitablematerials having the same or similar properties are also to be regardedas falling within the expression “carbon material”.

1. A secondary seal element including a base body made of a synthetic material, said base body comprising a base portion and a seal portion, said base and seal portions including coaxially aligned, axially adjacent through bores for the passage of a component, and an annular disc element accommodated in said base portion and including a through bore coaxially aligned with the through bores in the base and seal portions, said annular disc element being formed of a material which differs from that of the base body, wherein in the unloaded state, the through bore of said annular disc element has a radial dimension d which is greater than that D₂ of the through bore of said seal portion and smaller than that D₁ of said base portion of the base body, and in that the material of the annular disc element comprises a carbon material.
 2. The secondary seal element according to claim 1, that wherein the annular disc element is provided in a recess in an end face of the base portion and projects axially beyond the end face.
 3. The secondary seal element according to claim 1, wherein the synthetic material of the base body comprises PTFE.
 4. The secondary seal element according to claim 1, wherein that the seal portion comprises a pair of radially spaced resilient web elements having opposed outwardly directed sealing surfaces.
 5. The secondary seal element according to claim 4, further comprising means for radially expanding the web elements in a resilient manner.
 6. The secondary seal element according to claim 1, wherein the base portion has an essentially rectangular cross section.
 7. A mechanical face seal device comprising a pair of cooperating seal rings of which one is urged towards the other by an axial bias force and is axially moveably disposed on a sleeve, wherein for the purpose of sealing said one sealing seal ring with respect to the sleeve, a secondary sealing element according to claim 1 is provided in a thrust ring seated on the sleeve in axially moveable manner for transmitting said bias force, and wherein the sleeve is formed of a material having a coefficient of thermal expansion which essentially corresponds to that of the carbon material of the annular disc element.
 8. The mechanical face seal device according to claim 7, wherein the sleeve is formed of tungsten carbide. 